The present invention relates to an exhaust gas cleaner for cleaning an exhaust gas containing nitrogen oxides and oxygen in an amount larger than its stoichiometric amount relative to unburned components such as carbon monoxide, hydrogen, hydrocarbons in the exhaust gas, and a method of cleaning such an exhaust gas.
Various exhaust gases discharged from internal combustion engines such as automobile engines, etc., combustion apparatuses installed in factories, home fun heaters, etc. contain nitrogen oxides such as nitrogen monoxide, nitrogen dioxide (hereinafter referred to simply as "NOx") and oxygen in an amount larger than its stoichiometric amount relative to unburned components. The NOx is one of causes of air pollution such as photochemical smog and acidic rain, posing a serious problem of environmental pollution in a global scale. For these reasons, it is desired to remove NOx from exhaust gases emitted from various combustion equipments.
In the case of an exhaust gas from gasoline engines, NOx is usually removed by using so-called three-component catalysts. Also, in the case of large, stationary combustion apparatuses such as boilers, etc., ammonia is introduced into an exhaust gas, whereby nitrogen oxides in the exhaust gas are catalytically and selectively reduced.
However, in the case of an exhaust gas having a relatively high oxygen concentration such as those discharged from diesel engines and those discharged from gasoline engines operable in a lean condition, efficient removal of NOx cannot be achieved with the above-described three-component catalysts suitable for usual gasoline engines. Also, the reduction of NOx with ammonia introduced into an exhaust gas cannot be applied to movable exhaust gas sources such as automobiles, because this reduction system needs large apparatuses and poisonous, expensive ammonia.
Therefore, it has been desired to develop a method of reducing NOx in the exhaust gas having a relatively high oxygen concentration which is discharged from diesel engines, etc., and various attempts have been made so far.
For instance, there have been proposed methods of reducing NOx in an exhaust gas with a hydrocarbon introduced into the exhaust gas. One example of such methods is disclosed by Japanese Patent Publication No. 44-13002, which comprises passing an exhaust gas through a honeycomb-type ceramic filter carrying a platinum-group metal catalyst while controlling the temperature and flow rate of the exhaust gas, and adding a gaseous reducing fuel (specifically, methane, etc.) to the exhaust gas.
However, this method fails to efficiently reduce NOx in the exhaust gas discharged from diesel engines, etc. According to research by the inventors, even though a hydrocarbon having a small carbon number which is in a gas state in a normal condition, such as methane, propane, etc. is added as an NOx-reducing agent to the exhaust gas discharged from diesel engines, a large removal ratio of NOx cannot be obtained.
There has also been proposed a method of cleaning an exhaust gas, which comprises mixing an exhaust gas containing oxygen and NOx with a hydrocarbon to cause a reaction between oxygen and a hydrocarbon such that the hydrocarbon is partially oxidized to a reducing hydrogen gas and carbon monoxide and to lower the oxygen concentration, the resulting hydrogen gas and carbon monoxide being reacted with NOx in the exhaust gas, thereby decomposing them to nitrogen, carbon dioxide and water (Japanese Patent Laid-Open No. 49-122474). However, since the reduction reaction of NOx should be conducted at a relatively high temperature in this method, it is not suitable for cleaning the exhaust gas of automobiles.
There is an alternative method for reducing NOx which comprises adding a petroleum fuel as a reducing agent to a combustion exhaust gas in a high-temperature region alone or together with part of the combustion exhaust gas with or without air, and adding an air to the combustion exhaust gas in the downstream to reduce NOx in the combustion exhaust gas, the addition of the petroleum fuel such as methane, propane, gasoline, kerosine, naphtha, heavy oil, etc. being conducted by a plurality of steps such that a ratio of the amount of oxygen remaining in the exhaust gas to the amount of oxygen necessary for completely burning the petroleum fuel added is within a particular range (Japanese Patent Laid-Open No. 54-79161).
However, this method cannot remove NOx efficiently without keeping a region where a reducing agent and NOx are reacted at 1000.degree. C. or higher. Thus, this method is also not suitable for cleaning the exhaust gas of automobiles.
There has also been proposed a method of cleaning an exhaust gas, which comprises contacting an exhaust gas containing oxygen and NOx with a gaseous reducing agent such as hydrogen, and carbon monoxide. However, since a catalyst is not used in this method, a large amount of the reducing agent should be added. Accordingly, this non-catalytic method is effective only for the exhaust gas having a relatively low oxygen concentration, which is generated by burning nearly at a stoichiometric air-fuel ratio.
There have been proposed methods of reducing NOx by adding to an exhaust gas a reducing agent in a smaller amount than a stoichiometric amount relative to oxygen in the exhaust gas, in the presence of a catalyst such as zeolite carrying a transition metal (Japanese Patent Laid-Open Nos. 63-100919, 63-283727, and 1-130735, Thesis 2A526, 1990, the 59th Spring Conference of the Japan Chemical Society, Theses 3L420, 3L422 and 3L423, 1990, the 60th Fall Conference of the Japan Chemical Society).
However, the catalysts used in these methods are poor in efficiency when moisture is contained in the exhaust gas, and an actual exhaust gas contains about 10% of moisture. Accordingly, the methods using such catalysts fail to achieve a high reduction ratio of NOx in the actual exhaust gas. In addition, the optimum temperature of the reduction reaction of NOx is as high as nearly 600.degree. C. in these methods.
There has also been proposed a method of cleaning an exhaust gas, which comprises introducing methanol into the exhaust gas and bringing the exhaust gas containing methanol into contact with alumina at about 300.degree. C. ("Catalyst", Vol. 33, No. 2, 59 (1991)). However, this method is poor in efficiency of removing nitrogen oxides.